The most popular methods for ascertaining location of objects involve Global Positioning System (GPS) of earth-orbiting satellites. A receiver within a GPS positioning system receives signals from a number of satellites, and through processing of the satellites' signals that involve “triangulation,” the receiver is able to determine its position on the earth's surface with a great deal of accuracy. The degree of accuracy depends on the number of satellite signals that the receiver receives. The system is extensively used for navigation, and for locating humans. For human-locating applications, the GPS receiver typically includes a conventional radio transceiver that communicates to the base station the location that the GPS receiver has identified.
There are a few shortcomings to using GPS for locating of objects. Firstly, GPS does not work well indoors. Often the satellite signals do not penetrate through the building roofs, especially when those are made out of either brick or stone, and thus the receiver gets only few signals that are of sufficient strength to be usefull. Furthermore, even when the satellite signals do penetrate the building roofs, the accuracy of position estimation deteriorates significantly (30–100 feet). Applications that need better range accuracy thus have a problem. Secondly, GPS receivers take up to 30 seconds to initialize (before a reading can be obtained), unless they are continuously “on,” which is undesirable in small portable devices where battery life is an important factor. When it is continuously “on,” its power consumption is high and that limits performance and impacts price. Third, GPS receivers that hare sensitive enough to provide good accuracy are expensive.
Systems that employ RF signals and, more particularly, that employ the strength of a received RF signal to estimate the location of an object that emits the RF signal have also been proposed. Basically, the idea of these systems is to estimate the distance of the RF signal-transmitting source based on the strength of the received signal. However, when implementing such a system as a consumer product, it is highly desirable to employ a frequency band that does not require specific licensing, and that effectively means using one of the “free” frequency bands. In most cases, the transmitted power in such bands is limited by FCC Rules (in the US). Those Rules permit higher-power to be transmitted in the ISM bands, if spread spectrum is used. The simplest method to achieve spread spectrum is through frequency hopping.
US Patent application US2004/0039521 suggests the use of frequency hopping to increase the security of a child locating system. More specifically this application suggests that both the monitored station and the monitoring station have an algorithm that a seed should be sent by the monitoring station to the monitored station, and that this seed will cause both the monitored and the monitoring station to be synchronized to a common pattern of frequencies, in a frequency-hopping manner. Although an algorithm is mentioned, the only embodiment described is a table of frequencies, and presumably the communicated seed dictates the pattern of accessing the table.
US Patent application US2004/0036597 teaches another method, where security of the system is improved by using what it calls “rolling” identification codes that are continuously changing.
The above systems, like all other RF proximity detection system, are based on the same basic physical law that governs electromagnetic fields, where the signal strength decays in generally a known and predictable manner; i.e., in an inversely proportional to the distance, squared. Unfortunately, this is valid only for free space propagation and, in most practical cases that is not the case. Consequently, the received RF signal strength does not appear to strictly obey the above law. For example, multipath effects cause the received signal to significantly fluctuate. Often one would receive stronger signals for larger distances. In fact, in closed environment, the signal strength can fluctuate to the point that a method that is based strictly on the signal strength of a given received signal would be deemed useless. The problem of multipath effect is even more severe when the line of sight is obstructed (or partially obstructed—near line of sight) because in such cases the received signal is mainly a composite of reflected and refracted signals with substantially arbitrary signal strengths.
The main problem however, of utilizing such systems, is the inherent inaccuracy of RF signal strength as a base for range measurements, and none of the above-mentioned applications address, or even recognize, this problem